Method of treating protective coatings on magnesium and its alloys



mate or dichromate and obtained by subjecting the Patented Mar. 17, 1942 PROTECTIVE coarmes N MAGNESIUM AND rrs anno s Robert w. Buzzard, Kensington, Md.

No Drawing.

Application Serial No. 155,699

July 26, 1937, a

(Granted under the act of March amended April 30, 1928; 370 O.

The invention relates to the corrosion resistant coatings on magnesium and its alloys and has for one of its objects to increase the durability and the protective characteristics of the coating.

The method specifically relates to the treating of so as to not only dehydrate the coating but also to form a seal protective coatings on magnesium and its alloys with a chemical solution or seal in the presand surface coating, thus increasing thedura bility of the article ing its resistance to corrosion.

The invention disclosed and'claimed herein is an improvement on and extension of the subject matter of my copending application's, Serial 150,932,1lle'd June 29, 1937, now Patent No. 2,203,670 of June 11, 1940, and Serial No. 151,514, filed July 1, 1937, now abandoned.

The anodized article to be treated according to this invention is washed free of anodizing solution, providing the electrolytic bath is basically different from the chemical solution used in my method and is then immersed in a hot solution of chemical sealing compounds, preferably at or treated to wear and improvv 3, 1883, as G. 757) water.- Thus, the article in the hot bath will evolve gas vigorously on coming to the temperature of the bath. This action appears to be primarily adehydration of the film and, with this action, the liquid of the bath is absorbed by the coating. In practice, it has been found that the gas evolves for a short five to thirty minutes in a bath heated to 150 C. and then subsides. Thearticle should-remain in the bath'until the evolution of gas ceases. It is then removed and either forced dried or allowed to dry in the air. The bath may be heated to varying temperatures, slightly below, at, or above the boiling point of water, but must be limited to below the fusion point of the metal.

Specifically, as an example, if an anodic coating be formed on magnesium I bath of 10 percent dichromate and .2 per cent phosphate and the coating has been washed free of anodizing solution, its corrosion resistance will be greatly improved if it be first immersed for a period of about 20 minutes in a solution of 10 per cent dichromate and 10 per cent manganous sulphate 'heated to a temperature beabove the boiling point of water, but below the fusion point of the metaL. After suitable treatment in this solution, the article is removed, washedand then introduced bath of liquid comprising a. waterproofing or drying agent which, after the article is removed from the bath, is permitted to dry thereon.

The chemical sealing solution should contain a corrosion inhibiting ion such as either a chroa second salt as a reacting ion such as manganese, chromium, molybdate, borate, sulphate, phosphate, fluoride or a combination ofany of the said second salts. The concentration of the salts is limited only by their solubility, but may be varied to conform with time of application and quality of seal. Good chemical seals of coated metal have been coatings to treatment for periods varying from live minutes to one hour.

The bath of liquid comprising a waterproofing or drying agent should contain one or more materials selected from the following groups, namely, petroleum base oils, waxes, resins, synthetic resins or drying oils. Anodized and chemically sealed magnesium andits alloys, 2. bath of this type, have the peculiar property of evolving a comparatively large volmne of gas on heating to or above the bo ing Pa nt of directly into a hot tween 90 and 150 C., removed, washed and then introduced into a second bath of tung ofl heated to a temperature between 90 and 150 C. i The article is allowed to remain in the second bath until all evolution of gas has ceased, thus,

the time period is dependent upon the temperature of the bath. The material is then removed from the bath and allowed to drain and/or dry.

As further examples of compositions which can be used as chemical seals for electrolytic coatwhen placed in ings on magnesium the following are cited.

(a) 'l to 30 .per cent dichromate 0.25 to 10 percent (salts-of chromium) (b) 1 to 30 per cent dichromate 1 to 30 per cent borate (c) 1 i030 per cent dichromate 1 to 30 per cent sulphate (d) 1 to 30 per cent dichromate 0.25 to 20 per cent fluoride (e)- 1 to 30 per cent dichromate 1 to 30 percent phosphate (1) lto 30 per cent dichromate l to 30 per cent salt of manganese In the above compositions, chromate or a suit-' able corrosion inhibiting ion can besubstituted for dichromate and in various basic solutions, one or more of the following have been addedwith very good results, namely, chromium fluoride,

sodium fluoride, chromium phosphate, sodium borate, manganous sulphate, zinc phosphate, z'inc chromate, ammonium and other salts.

period of time, about or its alloys in a drocarbon-type as Santo-resins, the glyceryl phthalate type or alkyd type and the phenolic type as Bakelites.

(d) Drying oils including tung or China-wood oil, linseed oil, and fish oil.

While the aforestated chemical solutions with the waterproofing baths are described as seals. they may be used directly together to produce a combination corrosion resistant coating and seal on magnesium or its alloys. 7

The sealing method herein describednot only greatly improves the characteristics oi electrolytic coatings on magnesium and magnesium alloys, but also improves protective coatings produced by the various dip or immersion methods wherein the coating is produced by reaction with the magnesium. In other words, resistance against corrosion of protective coated magnesium or its alloysv can be greatly increased by treating the coated metal in a solution containing a corrosion inhibiting ion such as either chromate or dichromates in thepresence of one or more react-- ing agents selected from the groups of fluorides, manganese, chromium, molybdates, borates, sulphates and phosphates. This treatment may or may not be followed by immersing the metal in a bath containing a waterproofing agent. However, if both the chemical seal and the waterproofing bath are used resistanceagainst corrosion oi the coated metal is increased accordingly.

In comparative corrosion tests, the beneflts oi sealing were evinced in that coated articles oi identically the same type oi magnesium, com-,

prising dipped unsealed specimens, dipped chemically sealed specimens,'dipped specimens chemically and waterproofed sealed, unsealed anodized specimens, chemically sealed anodized specimens, waterproofing sealed anodized specimens sealed by the chemical solution followed by the waterprooflng bath, immersed in a tank containing 3 per cent NaCl solution, disclosed that the unsealed dip coated metal failed within 5 hours, the chemically sealed dip coated specimens and the unsealed anodized meta1 failed per cent within hours, the chemically and waterproofed sealed'dip coated specimen failed within hours, the chemically sealed and the waterproofing sealed anodised specimens withstood 300' hours, while the anodized specimens sealed by the chemical solution followed by the heated waterproofing bath were in good condition inexcess of 700 hoin's.

, The above mentioned substances, used in the a magnesium treatments are mentioned by way of hem! manued the "0 If" illustration and not limitation, since the invention comprehe'nds treatments of the indicated broadly above and is not to be limited except by-the appended claims.

The invention herein described claimed 7 may be used and/or manufactured by or for the Government of the United States of lime ca for governmental purposes without. the ayment of any royalties thereon or therefor. V

anodized specimens and y I claim: I 1. A method of producing corrosion resistant coatings on magnesium or magnesium alloys comprising anodizing the metal in a bath con- T taining the phosphate and chromate radicals, thepH of the bath being on the acid side.'subiectins the coated metal to a treatment with an aqueous solution containing manganous sulphate and a water soluble compound selected from the group consisting of the chromates and dichromates, and then immersing the metal in a drying oil selected from the group consisting of tung oil, linseed oil andflsh oil.

2.,A method of producing corrosion resistant coatings on magnesium, or magnesium alloys comprising anodizing the metal in a bath containing the phosphate and chromate radicals, the pH of the bath being on the acid side, subjecting the coated metal to a treatment with an aqueous solution containing chromium fluoride and a water soluble compound selected from the group consisting of the chromates and dichromates. and then immersing the metal in a bath of tuns oil heated to and maintained during treatment at a temperature substantially equivalent to or above the boiling point of water.

3. A method of sealing a protective coating on articles of magnesium or magnesium base alloys.

' produced by chemical reactionwith the metal, corrosion resistance thereof,

to increase the which comprises first treating the coated article and fluorides, the pH of such solution being on the acid side, and then immersing the treated article in a bath of a waterproofing agent se lected irom the grou consisting of petroleum base oils, waxes, res and drying oils at a temperature not substantially below the boiling point oi water, the latter treatment being continued until the evolution 0! gas from the treated article substantially ceases.

' 4. A method of producing protective coatinss of improved corrosion resistance on articles oi magne um or magnesium base alloys which comprises forming on the metal an anodic coating produced by reaction with the magnesium, coating to a non-electrolytic, treatment in a hot solution containing a water subjecting said soluble compound selected from the group consisting of the chromates and dichromates and also a water soluble salt selected irom the group consisting of the sulphates, phosphates, molybdatesrborates and fluorides, the pH of such solution being on the acid side, and then immersing the treated article in a liquid bath 0! a waterprooflng agent selected from the group consistingot petroleinn base oils, waxes,-resins and drying oils at a temperature not substantially below the boiling point of water, the latter treatment the watcrprooflns agent is a drying oil.

0. a method according to claim' 4 mam-s 6 the waterproofing agent is tung oil.

ROBERT w.-nUzzARD'. 

